1. Field of the Invention
This invention relates to paper machine productivity and means for attaining machine speeds significantly in excess of the prior art. The invention is concerned with eliminating stresses that act on the wet paper sheet as the web travels through the drying portions of the paper machine.
2. Prior Art
In papermaking, after sheet formation, the paper web, supported on one of a series of porous felts, passes through a series of press nips that mechanically express water from the sheet. The wet web at about 35-45% fiber content is then contacted with a series of heated drums or cylinders that evaporate water from the web to a finished dryness of about 90-95%. The web is, conventionally, unsupported at many points in the process as it travels between the later press nips and between the heated drums in the dryer section.
Machines that are not forming or drying limited are run at increasing speeds to gain production. A practical limit is always reached where increased productivity expected by further increases in speed is nullified by increased production losses due to sheet breakages and product defects. For example, newsprint machines appear to be limited to about 3,500 ft./min. (1070 m/min.) by current technology. This practical machine speed limit differs for each paper grade such as newsprint, liner, medium or fine paper. Further, within each grade of paper, the speed limit differs for differing basis weights.
Observations of operating paper machines show that, as speed increases, breaks in the web generally occur at those points in the process where the web is: (a) transported unsupported through the process while relatively wet and weak, such as occurs in transferring the web from the press to the drying section and between drying section rolls or cylinders, or; (b) required to change direction quickly while in adhesive attachment to a supporting element, such as occurs when the web is picked up by a felt from the forming wire.
When the speed of the machine is held constant, breakages increase with decreased paper basis weights within each grade. These breakages occur where the web is transferred from one machine element to another by pulling or peeling the web from the element to which it is adhered, such as occurs at transfers from forming wires to press felts and from press rolls to dryer sections.
For further discussion regarding drying section and press section stresses, see U.S. patent applications Ser. No. 091,684 filed Nov. 5, 1979 abandoned, now continuation-in-part, Ser. No. 234,288 and Ser. No. 091,212 filed Nov. 5, 1979 now continuation-in-part Ser. No. 252,969 respectively, both by Keith Thomas of Weyerhaeuser Company, incorporated herein by reference.
Edge "flutter" in the dryer section may also be observed. Flutter tends to cause edge "stretch," resulting in wrinkling defects in the finished product. Differential stretching at the web edges also imparts instability or "curl" to the finished paper.
It is well known that, as a paper web passes through the dewatering and drying process on the paper machine, it, in general, gradually develops strength with increased dryness. Practicalities determine that the overall speed of the paper machine be limited to make sure that stresses in the web do not approach, at any point, too closely to the paper web's breaking strength. Without a more detailed knowledge of the strength of the web and the stresses operating on it as it passes through the machine, papermakers have in the past attempted to avoid in an empirical way the increased sheet breakages observed with increased speed and decreasing paper weights.
These efforts include press and dryer section designs where the wet web travels with a porous felt or fabric during transit through at least a portion of either section.
Mahoney, in U.S. Pat. No. 3,503,139, provides a fabric intended to support the wet sheet throughout its serpentine travel from drum to drum in the dryer section. What actually happens as machine speeds increase is that the web is lifted and separated from its supporting fabric, particularly at points where the web approaches and departs drying cylinders. The lifting forces are centrifugal forces exerted on the web at certain locations in the machine and air currents caused by the turning drums and moving belts in the dryer section. These forces are generally non-critical in conventional systems only because these systems operate at low speeds. At higher machine speeds, however, these stresses increase in magnitude to cause breakages. Whenever the web is lifted from its supporting fabric, it is subjected to velocity stresses as if the fabric were not present.
It should be noted that the Mahoney web, as is typical of the prior art, is totally unsupported at the transfer from the press section to the first dryer cylinder. Thus, at this transfer, in addition to peeling stresses, the web is also subject to the velocity-related stresses noted.
In Mahoney, the web is, alternatively, partially wrapped in direct contact with one drum followed by indirect contact with the next drum. Mahoney compensates for the loss in heating effectiveness occasioned by the indirect contact of the web with the heated drum surfaces on alternate drums by operating those drums at higher temperatures.
In an improvement over Mahoney, Soininen et al., in U.S. Pat. No. 3,868,780, adds a number of rolls to the Mahoney system to guide the web into direct contact with each of the heated drums during transit of the web through the dryer section. In recognition of the increased likelihood of "flutter" separating the web from its support on the longer runs between dryer drums, the Soininen guide rolls operate under vacuum that adheres the web to their supporting surfaces. There is also an overall vacuum system to help hold the web onto supporting fabrics.
The Soininen system has a number of operating impracticalities. The guide rolls tend to cause a relatively large differential movement between the tender web and the fabric, resulting in "scuffing" damage to the web. The complexity of the system and extra components required introduce substantial capital costs. Operating costs are high because of the power required to drive the extra components and also since cleanout of paper after breakages appears to be difficult. Heat applied to only one side of the sheet, as in Soininen, results in paper products having different characteristics for each surface. These differences can cause printing nonuniformities when both sides must be printed.
In sum, the prior attempts to improve paper machine productivity by increasing machine speeds have generally failed because their designers have, up until now, had only an imperfect understanding of where in the papermaking process stresses operating on the moving sheet become critical and limit speed. Also lacking has been an understanding of how paper machine conditions, such as those affecting sheet temperature, for example, affect the ability of the sheet to resist velocity stresses.